Electronic ringing current generator



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Jan. 26, 1954 o. D. GRANDSTAFF ELECTRONIC RINGING CURRENT GENERATOR Filed June 13, 1952 [II on. fiEmDm 530m INVENTOR. OTHO D. GRAPDSTA FF BY 1 1 fl I, I

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Jan. 26, 1954 o. D. GRANDSTAFF ELECTRONIC RINGING CURRENT GENERATOR 4 Sheets-Sheet 2 Filed June 13, 1952 Q O N N QTHO D. GRANDSTAFF 'BY m mmw mom

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Jan. 26, 1954 o. D. GRANDSTAFF 2,667,632

ELECTRONIC RINGING CURRENT GENERATOR Filed June 13, 1952 4 Sheets-Sheet 3 I R 2' f0 0 Q 0 g 3 n INVENTOR. OTHO D. GRANDSTAFF ATTY.

Jan. 26, 1954 o. D. GRANDSTAFF ELECTRONIC RINGING CURRENT GENERATOR I 4 She''ts-Sheet 4 Filed June 13, 1952 mow INVENTOR. OTHO D. GRANDSTAFF ATTY - fier stages used to supply 1 stages of the unit.

Patented Jan. 26, 1954 ELECTRONIC RINGING CURRENT GENERATOR Otho D. Grandstaff, Oak Park, Ill., assignor to Automatic Electric Laboratories, Inc.,

Chicago,

111., a corporation of Delaware Application June 13, 1952, Serial No. 293,293

Claims. (01. 340-351) This invention relates to ringing current generators and is specifically directed to an electronic ringing current generator for use in a, telephone system, and having incorporated therein the ring back tone facilities, capable of supplying a series of ringing frequencies to a series of ringers individually responsive to a particular frequency.

An object of the invention is to provide an electronic generator capable of generating a series of ringing frequencies which are mixed with a ring back tone frequency and the composite signal is amplified to desirable proportions.

Another object of the invention is to use an oscillator for generation of the ring back tone frequency which is controlled and operated by a 4 modulating oscillator.

Another object is to provide suitable means for switching the ringing oscillator from one tuned circuit to another in such a manner as to preclude the accidental connection of two successive tuned circuits to cause the ringing oscillator to generate a sub-harmonic frequency.

Another object is to provide suitable means for switching the various frequency signals to their properly associated lines and to prevent the accidental connection of one frequency to more than one line.

A further object consists of the incorporation of a non-linear resistor responsive to voltage changes to eliminate transient voltages due to switching from one frequency to another.

Another object is to provide a ring back tone of such frequency and magnitude as to give a uniform aural signal to the calling subscriber and to eliminate the faults due to cross-coupling with other lines which are in close vicinity.

Another object is to provide a signal output from the unit which will remain essentially uniform in magnitude irrespective of the loading imposed thereon.

Other objects and advantages will appear from the following description and the accompanying drawing consisting of four figures.

Figure 1 shows the circuits of the three rectipower to the various Figure 2 shows the oscillator, amplifier, and the output stages of the ringing generator.

Figure 3 illustrates the ring back tone generator composed of two oscillator stages.

? Figure 4 shows the tuning circuits, the switching control circuits, and the motor unit.

The circuit shown in Figure 2 illustrates the components comprising the electronic ringing generator which was designed to provide alternating current at any frequency required to operate standard telephone-type ringers. The generator may be used to provide continuous ringing current adaptable for code ringing or to provide a single frequency of the standard multiple, non-multiple, and multiple-of-ten series of frequencies usually from 16 to 66% cycles per second. The unit is tuned very easily by means of separate sets of potentiometers associated with each frequency range.

When used as a continuous un-interrupted generator of a single frequency, no external control equipment is required to operate the unit.

' The output is connected to the exchange equipment through a standard ringing interrupter as required.

Whenever the unit is used as a source of ringing currents of various frequencies, a switching arrangement is utilized for changing the tuned circuits connections to the oscillator stage of the ringing generator. Any desired length of a ringing cycle may be used that is within practical limits. Five separate tuning circuits are contained in the unit. Each tuning circuit is adjusted in conjunction with the oscillator so that the combination is responsible for generation of a particular ringing frequency. The switching circuit is comprised of a series of relays, one per each frequency, and has a dual function namely, to connect tuning circuits to the oscillator stage, and to connect the output of the ringing generator to a particular supply lead which is individ ual to that frequency. These switching relays are periodically operated in sequential manner. Present modification ,allows each relay to re main closed for approximately one second before the next succeeding relay is operated. Thus within a period of five seconds, ringing currents corresponding to five frequencies are generated. Thls cycle of multiple frequency generation is repeated every five seconds by using a motor which makes one revolution every five seconds. A series of cams are mounted on the motor shaft which are distributed along its length. Associated with each cam is a set of electrical contacts which, upon closure, establish an operating circuit for the particular relay. The relay in question connects its associated tuning circuit to the oscillator and opens the preceding tuning circuit so as to prevent the oscillator from being connected to two tuning circuits at the same time should the preceding operated relay be slow in returning to normal. The relay also connects the output of the ringing generator to the parti-cular lead or equipment which is earmarked for receipt of that particular ringing frequency. As in the aforementioned operation, the relay similarly opens up the connection to the preceding supply lead so as to prevent accidental application of this particular ringing frequency to the preceding supply lead should the involved relay be slow ffto release. 7

The ring back tone generator comprises two oscillators. The modulating oscillator operates continuously at a frequency of 40 cps. The other oscillator operates intermittently at a fri-zduency of 400 cps. The modulating oscillator uses a twin tube and operates in the well known manner of the multivibatorftype. Whenever-a particular section of the twin tube conducts, the negative bias existing on the grids of the 400 cps. oscillator tube is overcom'e'and the oscillator is made to generate the 400 cps. signal. Hence it is seen that the output of the ring back tone generator consists of a' series of interrupted 400 's'ignalsg 'Tl iieoutputds thenie'dintothe buffefairiplifier"of"the"rifiging gherator'whe're 'ho'th signalsfthe ringing frequency and thering "hack "tone rfquency, are mixed togetherand "s'ibjs'eq'iientl'yamplified. H v

'rnegen'eratcr mp1 t leads for all frequencies newsflash-reaffirm the secondar of 'a single transform r. Taps may be available to provide "'a'nydesired'voltagefor each frequenc A'po'r- ""tion 'of the'output signal is "taken err from another secondary and fed"back' into the ringing "g'nratorffor the purpose orniatmtammg the auteur within a rengeior fpredetermined "values thereby prbvidingregulauen.

"The iind'esirable electrical disturbances which are created" by the switching operations involvi g the oscillator'and the tuning circuits are suppress byuse are nen-nnear resistor. This ftype'er resis'ter'is voltage'sensi'tive' and is suit- "able Tforsuppressing the transient 'voltage exf'c'ess'es by i o'fi'r ing; practically instantaneously, a

low 'iriipeda'rice path i resulting in "dissipation of "the energy. The resistor isof the type commoniyumawnas silicon carbide resistor. It is con- "nectedtobne of ithe inti'arrnediate stages of the "rin in gnerater wh're fit 'is "meet efie'ctive in "suppressing-the undesirable voltages.

The output is'coinpri'sed of alternating ourrents'which are sinusoidal in character and con- V ame d stray frequencies which have-a tendency to create operating difficulties due to cross-couplin'g with-- other-linesin proximity.

-A -detailed 'descr'iptionof this electronic ring- "in'g genera-termini" now be given. The commericial source of po'wr such as 1l5-volts,60 cycle A. is connected across the primary-winding IIZbf 'transformer I I I andjalso across the pri- 'marywi'nding H3 or transformer II4. Associated with the secondary windings I I5, IIB-and.

' I I8 of transformer III are two full-wave rectify- "ing circuit which together constitute-the main power supply I50. One of these'rectifiencirc'uits comprises two'full wa've high-vacuum rectifir tubes, I24 and IBIL-such as the commercial.

-5U4-G. The plates of tube I24 are'connected -in D multiple to "increase the power rating as alsoare theplates"of"tube I30. The plates of tube I24 "are connected to one side of the secondary winding IIB of transformer III-while the plates-of.

tube I30 are-connected to 'the opposite end of this-secondary winding I IS; the secondary'winding I I6 having --a I center-:tap I ll connected to "chassis ground over lead I08. Secondary winding H5 or transformer III- is connected-to the filaments of tubes I24 and. I30 and supplies heater current thereto. The D. C. output of this rectifier circuit supplies plate voltage for the final amplifier tubes 290, 293, 296 and 299. Inductor I2! and condenser I23 form a parallel resonant circuit and act to suppress the 120 cycle component of the rectified current output. The *odtpuijt is fiirt l'ier smciotlred by condensers I43 and I29. This is a low-impedance network filter --Which aids in maintaining a constant output voltage from no load to full load output. A resister W 42 =is connected across condenser I43 and serves as a bleeder resistor to discharge condenser I43 when the power is turned off.

secondrectifier circuit associated with the secondary w-indings of transformer III employs "ondary winding H6 of transformer IIi. Secondary winding H8 of transformer III supplies filament current to tu'be I3I. This second rectifier circuit supplies pl'ate' andscr'een'grid voltages for the driver stages 'of thissystem; plate voltage for the ring-back tone .generator andscreen'gr'id voltage for the'final amplifier tubes,"which components of this system willbeSllbSQurltly" described. The D; C.'ou'tput'of 'tubef'IB I is smoothed out "by the filter "comprising" inductor I 32 and condensers I33 and I34. Again, this "is"a low impedance type 'filter 'u'lh'ich niaintains good voltage regulation. "A "resistor I 35 is'con 'nected across condrlser isea'ne ser'y'esas "a bleeder resistor to discharge condenser "i3 3 when "the power is turne err.

Transformer I! t "is associa'ted"'with' a. second or summar ower" supply circuit 'I 2 5 which is used 'to 's'uppiwpiatereuage for the low power stages of this system. 'lhisd'solates the irequen'cy 'generatorfstages 'from"any effects of changes in outputlo'ad. Thls'power supply circuit I25 com rises'aYuu-waverectifier tube I26 which is similar to"there'ctlfied"tubes 24, I30 and- I3I. Thefplates'of 'tube" I26 are "connected to opposite ends'of theseccnaary winding lzii'oi transformer I I' l, this winding I 20 having" a'connection'from a centerta'p I 2| thereon through condenser I39 m-sarsuer-wi t both'resistors I40 and I4! to' chassis ground. "The filament onthis tube I26 is connected across secondary'wiiiding I22 which supplies heater currentthereto, and is also connected}tochassisground throu h Qindu'c't'oril 36 an {tiirougn'coudfenser m in paranei with" resistor I38. The rectified output of' tube I 28 is smoothed-by" the choke input "illter'cofisistof inductor Ieeandcaneeasefisi. "Resistor I38 serves as a blee'der resistor, discharging condenser I3? when thepower is turned off,'an"d also helps -to maintain a constant voltage output. The 'condenser I39-and resistors I40- and MI -in parallel therewith,=which are connectedbetween chassis group and the center tap I2! of-secondary winding- I 2 0. providea negativepotential-with respect to ground which isemployed as a fixed bias voltagefor the-final amplifier and oscillator tubes. Condenser I 39 se'ry es --to filter-the -=fixed bias voltage}arid;resis tors I00 and' IQI clisol-iarge condenser I39 when the power is' -turned off. Secondary winding-I23 ofgtransformer II I supplies-heater current for the tubes included -in -the -oscillator, buffer-ampli-fier-and phase inverter stag-es.

-The oscillator-stage23l comprises two vacuum tubes, 23'I--and- 244,'-'one-=of-which may be= the commercial GSN'ZGT' type--and the other the G-VBGT -type. Gnly onesection-ofthe GSNZ tube is used. These two tubes 231 and 244 and their associated resistors and condensers are'so connected as to form a Wien bridge type of resistance capacitance oscillator.

The cathode-plate circuit of the active section of tube 231 includes the plate resistor 232 and the cathode-bias resistor 248. The control grid of tube 231 is connected through condenser 239 to ground and is also connected through a pair of variable resistors connected in series, which are included in a tuning circuit to be hereinafter described, to ground; and is also connected through a second pair of variable resistors in series in the tuning circuit, and a condenser 24! to the plate of tube 244. The plate of tube 231 is connected to the control grid of tube 244 through a coupling condenser 235 in series with resistor 242 and variable resistor 241 including a tap 248 thereon. The control grid of tube 244 is also connected by the center tap 248 of resistor 241 through resistors 249 and 258 to the junction point of resistors I48 and I41, this circuit providing the fixed bias voltage for the tube 244. Resistor 241 is adjustable to permit adjusting the bias. of tube 244 to obtain the desired output. The plate circuit of tube 244 includes the plate resistor 243. A grid by-pass condenser 25l is connected from the junction point of resistors 249 and 258 to ground. The screen grid voltage of tube 244 is obtained from resistor 245, condensers 26l and 262 being screen grid by-pass condensers. The plate of tube 244 is connected to the cathode of tube 231 through condenser 236 and resistor 238 in series. Tube 231 conducts and the changes in the plate current thereof are reflected in the control grid of tube 244 through condenser 235 and resistor 242. Tube 244 amplifies these changes. A portion of this amplified signal in tube 244 is regeneratively fed back to the grid of tube 231 through condenser 24!, in series with the variable resistors in the tuning circuit, and the break contacts of a relay in the tuning circuit. The resulting change in the voltage on the control grid of tube 231 causes a corresponding change in the plate current thereof which in turn causes a change in the voltage of the control grid of tube 244 and a resulting change in the plate current of tube 244 which in turn results in a change in the control grid voltage of tube 231. A periodic motion is thus established and the frequency of oscillation in the output of this Wien bridge oscillator 23! will be determined by the setting of the variable resistors such as 428 and 42l in the tuning circuit 49 I. The resistance network of the tuning circuit and condensers 239 and 24I determine the phase and the magnitude of the feed back voltage. Another portion of the output of the amplifier tube 244 is degeneratively fed back to the cathode of tube 231 through condenser 236 and resistor 238 to improve the output wave form and increase the stability of the oscillator. The output of this oscillator 23l is connected to the control grid of the buffer amplifier tube 258 through condenser 246, resistor 251 and center tap 268 of resistor 259.

The buffer amplifier stage 252 comprises the left half section of tube 258 and the purposeof this amplifier is to serve as an isolating stage to prevent the final amplifier feed-back circuit from afiecting the oscillator stage 23L The cathode-plate circuit of this buffer amplifier section of tube 258 includes plate resistor 254 and cathode bias resistor 263. The output circuit of this buffer amplifier 258 is connected to the contro1 grid of the amplifier section of the phase-inverter tube 218 through the coupling condenser 256 and resistor 268. The phase-inverter stage 264 which serves the function of converting the output from the buffer amplifier 258 into two out of-phase components equal in magnitude for exciting the control grids of the driver tubes 218 and 282, comprises a two section tube 218, one section serving as an amplifier and the other section serving as a phase inverter. plate circuit of the amplifier section of this tube 218 comprises plate resistors 265 and 253 and cathode bias resistor 21l. The cathode-plate circuit of the phase inverter section of this tube 218 comprises plate resistors 266 and 253 and cathode bias resistor 212. As stated, the output voltage of the buffer amplifier tube 258 is applied to the control grid of the first or amplifier section of tube 218. The plate circuit of this section is connected through coupling condenser 214 to the control grid of one of the push-pull amplifier tubes 218, and is also connected through the condenser 214 and through resistors 215 and 216 to ground; the control grid of the phase inverter section of tube 218 is connected to the junction point of resistors 215 and 216. Thus, a portion of the output of the amplifier section of tube 218 is applied to the control grid of the phase-inverter section of tube 218. The plate of the phase-inverter section of tube 218 is connected by way of coupling condenser 213 to the control grid of the second push-pull amplifier 282. When the output voltage of the amplifier section of tube 218 swings in the positive direction, the plate current in the phase-inverter section of tube 218 increases, thus increasing the voltage drop across the plate resistor 266 which in turn causes the plate voltage of the phase-inverter to swing in the negative direction. Thus, when the output voltage of the amplifier section of tube 218 swings positive, the output voltage of the phase-inverter section swings negative and is therefore out-of-phase with the output voltage of the amplifier section of tube 218. Resistors 215, 216 and 28I are of such values that the voltages applied to the grids to the pushpull amplifier tubes 218 and 282 are of equal magnitude.

The driver stage 211 of this electronic ringing generator employs two beam power amplifier tubes 218 and 282 such as the commercial 6V6GT tubes. These tubes are connected in push-pull, their plates being connected to opposite ends of the primary winding 284 of transformer 283, the center tap 285 of which is connected to the source of plate voltage, and their control grids ar excited by signals equal in magnitude but 180 out of phase as previously stated. The plate-cathode circuit of tube 218 includes the cathode bias resistor 288.

The push-pull method of operation gives increased power output and eliminates distortion due to even order harmonics and hum caused by plate-voltage supply fluctuations. Transformer 283 has a step-down ratio to provide a low impedance driving source for the final amplifier tubes which take grid current at heavy load conditions.

The final amplifier stage 288 comprises four beam power amplifier tubes 298, 293, 296, 299, such as the commercial 6L6G tubes, connected to give a parallel push-pull amplifier circuit. The control grids of tubes 298 and 296 are connected to one end of the secondary winding 286 of transformer 283 and the control grids of tubes 293 and The cathode 3&625632 "ing "connected-to the 'sonrceriffixed "bias voltage at the. auxiliary power .supply f25. "volta siequal magnitude, I180 'out-o'ffphasc,farei'silpi lied tothe control grids .or each; parallel. combination,

. tubes290 and Y196 operatingf'in parallel andtubes 293 and'293operating in parallel. 'TBy-pass .c'o'ndensers 29 I 292," 2 9 1 .:anid'29'8 "'serve to "suppress any high-frequency components. 'Resistors 23 9, 234,"2B51and 220 are :plamio equalizing 'resistors. .The sine waveoutput'a'ppearsacross the secondary. winding 222 .of output transformerg'22 i A second secondary winding'223iisiincludediin 'transformeffl liandserves toffurnish anegative :feedbabk voltage. to; theijgrid of the 'amplifiensectionof tubeZ Iflf;in"the phasefeinverter stage'i264 through resistorv 269. :As the output load increases, orfdecreas'es. a'correspondingzincrease .or

- decrease in amplification 'w'ill take place' in the amplifier section. of tube 210.. resultingdn an'increaseor decrease in theoutputvoltage attransformer 22!. "Thu,s,'the output .'voltage is"held automatically at .a constant valueas the load changes overa wide'range.

This embodiment of the. electronicv rin .gen- .erator was intended for .use in supplyingminging current for-telephone.exchanges,' anda ringback itone generator'3201is also provided to supplyfa single. frequency io'f;.uniform. rn'a'gnitu'de?forv ringback tone .to .-a calling .subscriber. Ringiba'ck ltone generatorLSZO. supplies intervals of. 400,.cyCle signals superimposed onlthei ringing signal atlithe output transformer? l. .T'Theringbacktone generator Y 32 0. comprises -two iosc'illator -circuits,j3'2 I -andf3 2 2 .one producing. oscillations. air-.40. cycles .per. second. andithe .other .producingbscillations at'400. cy'clesrper second. .ITheA'OOcy'cleoscillaLtor 3 22 .in'c1udes..the tube 339. such as-fiSNTGlcone LpIateof which. is. connected .to. one end. of the .primary winding? of transform'er' 3M and. the eother plateof which. is. connected -.to; the other end, of primary winding?!" and 'bothlleading through .a .center .Ltap T343 .and .plate voltage idropping resistor; 324;.to thesoureeofeplate voltage. "The plate-scathoiie.ircmtsmf this .tfibel 339 include thecathddel' bias -resistor"34'0. 'lThe con- 'trol' grids of this tube 339 are conneotedltoopposite. ends .of .the.secondary -winding.. 331. .of transformer 336,1Tthe center"tap7338..6f.which is..con- .nectedlthroug'h; resistors? 33 5. and- 33 3 .to r a source of, negativepoter'itial. "This '400.cycle-.-oscil1ator T533 9': is normally" inoperativedue'tolthis. negative vpotential on 'thegrids.

"The 40 cycle. os'i1lator32 I comprisesLtiibeZBflU such as6SN'7GT. IOne platethereot is connected throughgplate resistor "325 and the. other plate thereof is connected,through.plateresistorjw, and both connectedto. a source of.plate voltage throughplate voltage dropping resistor-I323. {The control. grids thereof. areconnected. to .a .source of negative biasing potential...through resistors '33l andj332. l'The'40 cycle .oscillators32l is normally. operating. 'I 'Ihis modulating oscillator '32 I is of the well-known multivibratorjtype wherein Ollie section. of the tube 330.. conducts whilecthe "other section of the same. tubelbecomes biased 6ft. ""Os'ciliations are started'by momentarycunbalance, say a more. positive voltagepexisting on "the L 1(l'eft section of tube 33 than-on the. grid '.of.'R'(i'ight) sectionof sametube. 'IIThis. voltage 'isamplifiefd and reappearsiat. the..L.grid-to be *reamplified. "Ifhis is manipulative, an'dlinstan- 'taneous action so ithat" the"Lj grid. risesiabruptly '"toa positive value;whfie'ithe'fi'grid' poteiiti'al'just .assuddenlybecomes morenegativethan thecut- Toff value. Asaresult amplification ceasesjiand L' triode of tubetstt draws af'hed'ilyhhte current while R triode' takes no ,plate current. 'jBilt this situation is only,- momentary 'because'the leakage through the grideleak resistance 332 .of i; the .R ftrioj'cle' brings thegri'dpotential on grid of R'ftriojde "back to zero. Whenthenegative potential-on grid of -"R 'trioidel has dropped. sufiiciently so "that amplification is possible, some minute voltage "will istart,..arnp1ific aition in the R triode, operation of which is reverse ;to.fthat.,previouslydescribed for .the'L :triolde. -Whenever .JL 't'riode -conducts, plate of. the 1R triodev doesinotdraw any. current thereby resulting in a high positiv potentialeat .point32'l which is impressed therefrom across re- ,sistori32 8, condenser 3'29, resistorj33 5 to jtheicen- "ter-tapl338 .of the. secondary. windingk33'1 then zito'the grids-of .tnbe1339. This. causes tube'f 339 together .With its associated components tolbreak -intoiosiiillations ofJlOO. cycles persecond, .theoutput o'f.which is taken from the secondary winding 344 or .theoutput. .trans'formerffll .via. the variable resistor. 345. ',This.-modulated;400 cps. signal .a'ffectsthe current flow. throughthe cathode re- ,si'stor263in the buffer amplifier 252 causing the .signal .to be mixeduandiampiified together with -th ringing. frequency, .Zthe combination of which appears at the. output trans'formenZZl. ,-As soon asthe Ltriodeof.tube.330 .ceases.to conduct, the

R'triodel begins, to conduct and the positive voltage atpoint 321 "dropsjtosuch anextentthatthe ,gri'ds on. tube I 33 9 vassume :a" bias whichprecluoles 'furtheroscillation of tube '339. .-The result .of the .35 Operation. of thev ring back Itonegenerator 332 this ..that ,ajseries of discontinuous spurtsconsistingnf 400 cycle. per. second oscillations are. impressed 1,1pon tl1eR grid of ithebuffenamplifier.tube 258. .The rate .of occurrenceof these spurts .is-gov- 4U erned J by .the. modulatingoscillator.32l which correspondsto 40. cycles ,per second. .The above operation is analogous to .i that known in ,CW 'wireless telegraphy.

This .i articular.embodiment. of the invention is designed for use in -ia'telephone exchange where usually 5 different. ringing. frequencies are reouired such. as-16"-/3,f 25,, 33 ,,.50.and 66% .cycles per second. [In the'telephone exchange the ringingffrequency isiintermittently-connected .to the telephone'linesffor a periodofone second out of everyjdseconds. A frequency selector andout- -put. control circuit are provided. and comprise a motor .4 [I05whi'ch turns at the rate of one revolution -every j'five seconds and ..5 cams JIM-405 "fixed1y..mounted on jthermotor shaft 406 with which are associatedfive pairs. of .contact Springs 464- 468. respectively. .The cams aresoarranged asiltofclose thespring. contactssuccessively in one .secondperio'ds, each pairv of contactsbeing closed 0 onceipereach five-second period. .These pairs of .springconta-cts464-458,. whenv closed, causeasso- .ciatedre1aysA45e449.to operate. .These relays .operate. successively and sin; 8601181168 during-a .ifi eseconde-period totune the. electronicringing a5.. generator to produce..-the.sfive .diiferentminging "frequencies and to close the: outputicircuit :tothe exchangeeq ipment.

To Ifillustrate .how the ..various components cooperate .in producing. a desired result, a single .operationwill .be indicated. The protuberance ,on.the. cam 402 is in its ruppermostipositionra sulting in. closure of .contacts465. A a-result, .relayi MG; is, operated over the following .path: ground through contacts'465 and through'the *winding .onj'relay 446 to :ba ttery. Make...con-.

tacts 413 close so thata first portion of the oscillatory circuit is established for generation of 25 cps. over the following: the left grid of tube 231 over the line 2! I, line 4| I, through the break contacts 419, through the break contacts 416, through the break contacts 414, through the make contacts 413 on relay 446 over the line 4H5 through the fine Vernier potentiometer 423 through the coarse Vernier potentiometer 424, through the resistor 425, over th line 4|4, over the line 2M through condenser 24! to the plate of tube 244. Contacts 412 open and prevent accidental paralleling of the previous 16% cps. oscillatory tuning circuit to the present 25 cps. tuning circuit should contacts 411 be slow in opening up. The second portion of the oscillatory circuit is completed when make contacts 454 on relay 446 close and establish a path from the left grid of tube 231, over the line 2| I, over the line 45 I, through the break contacts 439, through the break contacts 481, through the break contacts 485, through the make contacts 484, over the line 493, through the potentiometer 429, through the potentiometer 436, through the resistor 43!, over the line 408, over line 203 to the cathode of tube 244. Contacts 483 open and prevent accidental connection to the previous tuning circuit should the make contacts 482 on relay 445 be slow in opening. Make contacts 453 on relay 446 close and allow the 25 cps. frequency signal generated in the electronic harmonic generator to be sent over to the desired subscriber equipment. Contacts 452 and contacts 454 open so as to prevent accidental application of the adjacent frequencies, 16 cps. and 33 /3 cps. over the 25 cps. line. It is to be noted that contacts 452 will break first before contacts 453 will make so as to prevent application of the 25 cps. frequency sig-' nal to the previous line which is to receive only the 16 cps. frequency signal. Operation of the other components comprising the other ringing frequency circuits is similar to that described for the 25 cps. case.

It will be realized that as the cams make and break energizing circuits to the various relays, the latter in operating and releasing will cause a distinct break between the oscillation periods. Although this is of short duration, the electrical disturbance created during the switching operation from one tuning circuit to another is of such magnitude as to be painful and irritating to the aural sense of the calling party, and to introduce interference in other circuits in vicinity by the cross-coupling phenomenon. To overcome this undesirable element, a nonlinear resistor, of the type known as silicon carbide, is used to suppress any transient voltages arising out of this switching operation. This non-linear resistor 219 is connected between the two grids of tubes 210 and 282 as follows: the left grid of tube 210 through condenser 261, through condenser 214, through the resistor 219 and to the grid of tube 282. This type of resistor 219 exhibits a high resistance to the ordinary operating voltage, but as soon as a high transient voltage appears across the resistor, it exhibits a low resistance. This drop in resistance upon appearance of abnormal voltage is not linear but is logarithmic with respect to the voltage and presents an effective method of combating the undesirable characteristics of transients.

While there has been described what is at present considered to be the preferred embodiment of the invention it will be understood that various modifications may be made herein and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention. a

What is claimed is:

1. An electronic generator for connection to a plurality of ringing devices each responsive to a different frequency, comprising a plurality of circuits each tuned to a distinct ringing frequency, actuating means, means operated in response to operation of said actuating means for switching in cyclic succession the ringing generator from one tuned circuit to another and for switching the output thereof to said ringing device, said ringing generator operable responsive to operation of said switching means for generating a particular ringing frequency associated with each tuned circuit when connected thereto, means for generating a ring back tone signal, control means operated by said ringing generator for controlling the operation of said ring back tone means, means effective in response to receipt of said ring back tone signal for superimposing said signal upon each ringing frequency signal, means for amplifying the composite signal, and means for suppressing the transient voltages due to said switching operations, said suppressing means effective in response to receipt of said transient voltages to suppress said voltages whereby the composite output is free of deleterious foreign frequencies.

2. In a combination with an electronic ringing f generator including a plurality of tuning circuits each operable at a difiierent frequency, means for sequentially switching the ringing generator from one tuning circuit to another in an established periodic manner, said ringing generator operable responsive to operation of said switching means for generating a signal of particular frequency corresponding to said connected tuned circuit, means for generating a low frequency signal,

other means for generating a high frequency signal, said other mans inetrmittently operated in response to operation of the low frequency generating means for generating an intermittent high frequency signal, means operable responsive to receipt of said particular frequency signal and said intermittent high frequency signal for combining and amplifying said signals whereby the output is comprised of the two frequenc1es.

3. In a combination as claimed in claim 2, means for suppressing transient voltages due to the switching operation, said means effective in response to receipt of said voltage disturbances to suppress and modify their effects whereby the output is essentially free of any undesired voltage variations.

4. In a combination as claimed in claim 3, means for controlling the output of said ringing generator, said means effective in response to a drop in said output for bringing the output back to normal and said means also effective in response to a rise in said output for bringing the output back to normal whereby the output remains substantially uniform during switching to various external equipment.

5. An electronic ringing generator comprised of a plurality of stages serially common to a network of tuning circuits each of said tuning circuits tunable to a different ringing frequency, means externally operated for switching sequentially from one tuning circuit to another in a repetitive manner, modulating means, ring back tone means, said ring back tone means operated in response to operation of said modulating atone 3e" 1'1? means;saidmodulatinggneanstefectii eiimmodulating a ring' back'tone signaligeneratediby the ring back tone means, means" operable resppne sive to operation of said. ringing, generator. for introduction of said modulatedsignalinto one offth'e stagesof said' electronicringingrgenerator, means operated'in.response to receipt of said. signal for simultaneously mixing; and amplifyin'glboth the ringing signal and the ring backtone signafwhereby theoutput of Lthe ringing, generatoro is comprisedlof'Jtwov frequencies; ,a ringingfrequency and'ja ringbacli ,tone frequency. v

61111 acombination asiclaimed' inclaim; 5, suppressor means connect'eclito at least one of said stages', said" suppressor means. effective in re? sponse tomeceiptofvoltage. transients due to. the. switching operation for suppressing said transients whereby the output consists of" the ringing frequency; and? the ring back tone. frequency, signals. substantially; devoid; of I the VOlte age, disturbances.

'7'. In a comlciinationv as cl'airn'ed;in. .c1aimj.6'; means 'for controlling the .output' signalj so that it remains substantiallymfj uniform magnitude; said'fmeans efiect'rve'infresponse to a drop intl'iee output 'forbringinggthe output back to its iormeri status; ,t saidl means also.-,. effective. ir'n responses to a rise in thejoutput for, bringing the output back,

to its former status whereby, the:outp 1t..of;said.. ringing. generator; remains.v substantially uniform regardless ofjthe. degreev ofiloadingn sun. a combination with" an; electronic. ampli-I fier comprised of a plurality, of. stages oner oil saidsta'gesreceivingjnput from a. source .of varn able frequency, signal's mean's for generating: a

modulatingsignal', other means for generatingua distinctfrequencx signal, saidother means operoperation" of'onse-of"said stages for *generatingw;

ated' intermittently in response to continuous.

operationoffsaidfmodulatinggmeanstfonintlodifmgtionsoi said? modulated." distinct'frequenc signal" intooneof said'stagges, means 'operableresponsive to receipt off the input" signal and; saidi modulated signal for mixing, and amplifiringsaid signalswh'erebyflth'e outputconsists .of a combination of the twofrequencis.

9; In a combination'as claimed 'in claim 8", sup pressormeansisensitive'to ,voltage'magnitudes de- V viating from. thenormalvoltage of said" input. signal; said, suppressor'means" connected; to at least one of? the stages of 1 said amplifi'enysaidi suppressor means efi'ective in responseto receipt' of voltage deviationsv from said 1 normal signal" voltagev for' suppressing: the voltage abnorma1i-, ties to keep theoutpufisubstantially free of-said. abnormal voltage variations:

10;; An .electronicoringiba'ck tone generator com?- prised of a plurality ofoscillator stages each opera able at a different frequencyymeans responsive to low frequency signaijgneansin anotheristageiope erated-inresponse to: receiptfofoni'y one-half of each alternation ofsaidlow frequency 'signa'liffori generating'a highfrequencysigna'l whereby the" ring 'back.tone"'consist's'of ailiighfirequency signal modulated byth'e" low -freonmncy signal;

OIlZI-IO D." GRANDSTAFF.

References Cited in the file-0f this patent STATES: EYXTENTS Number: Name": Date 2-,457,131. Curtis.. ,..Dec: 28; 1948-7- 2;502,68'7 Weinem v A-pr. 4 11959 2,561,747. MerrillgoJrz GU53]; .July 24; -14 2581:1156: Walmsl'ey et- .al'. Jan; 1; 1952' 2585,0225 L'ewis: eEeb; 12;- 19521. 

